Filter material and process for producing No2 -free gases or liquids

ABSTRACT

Polyarylene ethers are employed as filter material for removing NO 2  from gases and liquids. In the presence of an oxidizing agent having a redox potential of at least 0.96 V SHE, NO can also be removed. The filter material is used to produce NO 2  -free gases, for example in the medical sector.

The invention relates to a filter material, a filter and a process forproducing gases and liquids which are free of nitrogen dioxide.

DESCRIPTION OF THE PRIOR ART

DE 43 28 450 A1 describes a filter material and a process for removingoxides of nitrogen from gases and liquids. Polyarylene sulfides areemployed as polymeric filter material for nitrogen dioxide (NO₂). In thereaction of NO₂ with polyarylene sulfide, the sulfur groups areoxidized, reducing NO₂ to NO in the process.

NO₂ -free NO gas or NO gas mixtures are required in the technology ofwaste-gas measurement in order to calibrate measurement and analysissystems.

Recently, the medical use of nitrogen monoxide (NO) has acquiredparticular importance. In the case of patients with severe pulmonarydiseases, the measured addition of NO to the air supply for breathingcan reduce high blood pressure in the lung circulation. In conjunctionwith the bronchodilatory effect of NO, this results in improved airsupply to various sections of the lung, and thus to improved gasexchange as well. Important factors in this context are the exactsetting of the NO content and the minimization, extending toelimination, of the NO₂ content. This problem occurs to an increasedextent in the abovementioned application, since in said application NOis mixed with atmospheric oxygen at temperatures of around 40° C. and athigh atmospheric humidity and passed over distances of about 3-6 m.During this procedure, some of the NO will be converted to NO₂. Thisleads to an increase in the NO₂ content of the gas, which is damaging tothe patient, and to a reduction in the NO content by the amount of NO₂formed. The desire here is for a converter which transforms the NO₂formed back to NO, shortly before it is breathed in by the patient, butwithout any other alterations to the gas mixture, for example itsmoisture content or temperature. Compared with an NO₂ filter, thisconverter would have the advantage that there would be no change in theNO content originally set.

The colorless nitrogen monoxide (NO) reacts rapidly with molecularoxygen to form brown nitrogen dioxide (NO₂). In the presence of air orwith ingress of air, therefore, NO₂ is formed from NO. Consequently,owing to the omnipresence of oxygen, NO₂ is an inherent impurity in NO.With the medical use of NO in particular, toxicity dictates that thecontent of NO₂ must be very low.

SUMMARY OF THE PRIOR INVENTION

It has been found that it is possible to remove NO₂ from gases orliquids by bringing them into contact with a polyarylene ether.Furthermore, the selective removal of NO₂ from NO or NO-containing mediasuch as gases or liquids can be achieved with the aid of a polyaryleneether. Polyarylene ethers are therefore suitable filter materials forNO₂. This is surprising since there is a great difference in structurebetween the polyarylene ethers and the polyarylene sulfides, the formercontaining no sulfur groups.

The invention provides a filter material for removing NO₂ from gases andliquids, which comprises a polyarylene ether.

Furthermore, the invention provides a filter for removing NO₂ from gasesand liquids, which comprises a polyarylene ether.

The invention additionally provides a process for producing NO₂ -freegases or liquids, in which the gas to be purified or the liquid to bepurified is brought into contact with a material which comprises apolyarylene ether.

NO_(x) is used as a collective term for NO, NO₂ and N₂ 0₄.

NO₂ and N₂ O₄ are in chemical equilibrium. N₂ O₄ is removed from gasesand liquids by polyarylene ethers.

The expressions "free of NO₂ " and "quantitative removal of NO₂ ^(1")denote that the content of NO₂ in a medium is less than 1 ppm.

The filter material, the filter and the process of the invention arealso suitable for removing NO₂ from NO-containing gases or liquids.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 of the Drawing is a schematic illustration of the use of a filterof this invention.

FIG. 2 of the Drawing is a schematic illustration of a breathingmask/filter combination of this invention, wherein the filter acts uponthe intake air flow for the mask.

A polyarylene ether is a polymer which comprises at least one aryleneether unit (--A--O--; A is arylene). An arylene is an aromatic unithaving two bonding sites, e.g. --C₆ H₄ --. Mono- or polycyclic aromaticcompounds can form an arylene unit, such as benzene, pyridine,naphthalene, phenanthrene or anthracene. Substituted arylene units arepreferred. Examples of arylene substituents are C₁ -C₁₈ -alkyl, such as--CH₃, --C₂ H₅, --C₃ H₇, --C₄ H₉, --C₅ H₁₁, --C(CH₃)₃, --CH₂ --CH(CH₃)₂,--C₂ H₄ --CH(CH₃)₂ or --CH₂ --C(CH₃)₃. Suitable polyarylene ethers aredescribed in the as yet unpublished European Patent Application havingthe file reference 95112259.7, filing date Aug. 4, 1995, bearing thetitle "Filter material and process for removing ozone from gases andliquids", to which reference is made. Now EP-A 697,236

A preferred polyarylene ether is poly 2,6-dimethylphenylene oxide!.

The polyarylene ether can also be mixed with one or more other polymers.

The polyarylene ether can also be a block copolymer or a blendcomprising at least one polyarylene ether. Examples of suitable blendsare polyarylene ether blends which comprise polystyrene homopolymerand/or polystyrene copolymer and/or polyamide and/or polyolefin.

Examples of polyarylene ethers and their preparation are given inUllmann's Encyclopedia of Industrial Chemistry, 5th edition, volume A21,B. Elvers (Ed.), VCH, Weinheim-Basel, Cambridge-New York 1992, heading`Poly(phenylene oxides)`, page 605-614, to which reference is made.

Polyarylene ethers and polyarylene ether-containing polymers arereferred to below as polymer.

The polymer can be used, for example, in the form of powders, granules,fibers, nonwovens, woven fabrics, films and/or shaped articles. Thepowders possess commercially customary particle sizes, with granulesbeing a further possibility for use. The important factor in thiscontext is that the liquid or the gas to be treated can be passedthrough the powder in the form, for example, of a fixed bed withouthindrance. If the polymer is used as fibers, the fibers employed arestaple fibers, needle felt, nonwoven material, card sliver or wovenfabrics. Films or film scraps in a suitable form can also be used.

The polymer can generally be employed as unadulterated material.However, it is also possible to add fillers, such as chalk, talc, clayor mica, and/or fibrous reinforcing agents, such as glass fibers andcarbon fibers, whiskers, and further additives and processingauxiliaries, for example lubricants, release agents, antioxidants and UVstabilizers.

Coatings of support materials with the polymer can be obtained byapplying solutions of the polymer (examples of solvent being toluene,chloroform) to the support material. Impregnated forms are produced, forexample, by impregnating an absorbent support material. The supportmaterials employed are generally inorganic substances such as glass,silica gel, alumina, sand, ceramic compositions and metal and organicsubstances such as plastics.

It is also possible to apply to the polymer substances such as metals,especially noble metals and transition metals, or metal oxides such astransition metal oxides. The metals or metal oxides can be applied tothe polymer, for example, by impregnation, and are then present in theform of small clusters, for example.

The process of the invention can be carried out at any temperature belowthe softening point of the polymers used. In general, the servicetemperatures are in the range from minus 10° to 200° C., preferablybetween 0 and 180° C.

The time required for contact of the medium to be purified with thefilter material depends, inter alia, on the flow rate, the residencetime, the surface area of the filter material, the geometry of thefilter and the temperature. In general, the time of contact of thesubstituted polyarylene ether with the medium to be purified is in therange between 0.001 seconds and 10 minutes, preferably between 0.01seconds and 1 minute. These times can also be exceeded, however.

The magnitude of the specific surface area of the polymer has a markedinfluence on the filter action of the polymer. The filter action of thepolymer is generally greater the greater the specific surface area ofthe polymer. A large specific surface area and porous structures of thepolymer, with a harmonized ratio of micropores to macropores provingfavorable, are particularly advantageous for a filter effect. The filtereffect of the polymer is also influenced by the crystallinity or,respectively, the extent of the amorphous fraction of the polymer. Ahigh amorphous fraction in the polymer generally promotes the filtereffect of the polymer.

The removal of nitrogen dioxide from gases or liquids by contact with apolyarylene ether may be based on a chemical action, catalytic actionand/or physical interaction. In the case of chemical action, thepolyarylene ether reacts with the nitrogen dioxide and in doing soundergoes oxidation. In the case of alkyl-substituted polyaryleneethers, the alkyl group is oxidized. This takes place with particular-readiness in the benzyl position, i.e. on the carbon atom of the alkylgroup of an arylene unit which is adjacent to an aromatic nucleus.

In the removal of NO₂ from gases or liquids, the polymer does not formany volatile products.

The removal of NO₂ can be applied to NO_(x) -containing gas streams andliquids. The process of NO₂ removal operates, with gases for example,with a NO content of between 60% by volume and 1 ppb, preferably between50% by volume and 10 ppb and, with particular preference, between 40% byvolume and 50 ppb. The NO₂ content which can be separated off is between50% by volume and 1 ppb, preferably between 20% by volume and 10 ppband, with particular preference, between 10% by volume and 10 ppb. Theratio of NO to NO₂ in the liquids or gases to be treated can in thiscontext be between 1,000,000:1 and 1:1,000,000, preferably between10,000:1 and 1:10,000 and, with particular preference, between 1000:1and 1:1000.

The filter can contain the polymer, for example, in the form of a powderbed, a nonwoven, a nonwoven/powder mixture, or a lattice or honeycombstructure. The powder can, however, also be incorporated into nonwovensformed from other materials.

The filter material, the filter and the process for removing NO₂ aresuitable, for example, for producing NO₂ -free NO gas or NO₂ -freeNO-containing gas for analytical gases and, in particular, for NOapplications in medicine, for example in the case of patients withsevere pulmonary diseases, the measured addition of NO to the air supplyfor breathing can reduce high blood pressure in the lung circulation.This applies to patients both with IRDS (Infant Respiratory DistressSyndrome) and those with ARDS (Adult Respiratory Distress Syndrome).Also conceivable, however, are applications in cardiac surgery, for theintensive artificial respiration of patients with NO in order to reducehigh blood pressure in the pulmonary circulation. Important factors forthese applications include both the exact setting of the NO content andthe minimization, extending to elimination, of the NO₂ content.

Owing to the harmfulness of NO₂ to humans, the formation of NO₂ from NOand oxygen in medical applications of NO, which employ mixtures of NOand oxygen at elevated temperature (e.g. 40° C. ) and as amoisture-containing gas, is a great problem. The use of the process orfilter of the invention for removing NO₂ can eliminate this problem. NO₂produced between filter and lung can no longer be removed. The distancebetween filter and lung should therefore be as short as possible.

The process and the filter for NO₂ removal can be employed, in medicaltechnology, at a number of points in an artificial respiration system.The filter can be accommodated directly behind the pressure reductionvalve in order to minimize the proportion of NO₂ which is formed orwhich remains during the production of the NO/nitrogen gas mixture used.Using the process described herein, it is possible to purify gasmixtures which contain NO in a concentration of between 1 ppb and100,000 ppm in nitrogen, preferably from 1 ppm to 10,000 ppm. In thiscontext, the volume flow of the-NO/nitrogen gas mixture can be between0.001 and 1000 l/min, preferably between 0.01 and 250 l/min.

In the case of treatment involving NO uptake via the lung, theNO-containing gas and the added air can be purified upstream of or inthe filter, with the result that an NO₂ -free gas mixture is inhaled. Inthis case the filter can, for example, consist of a breathing mask, withthe filter containing the polymer being inserted in the intake air flowof the mask.

When the polymer comes into contact with nitrogen dioxide, nitrogenmonoxide is sometimes produced. The filter effect of the polymer withrespect to nitrogen monoxide is negligible. Nevertheless, nitrogenmonoxide as well can be removed quantitatively if at least one oxidativeinorganic or organic compound which has a redox potential of at least0.96 V against the standard hydrogen electrode (SHE) is added to thepolymer or filter material, examples of such compounds being chlorinatedlime, sodium hypochlorite, vanadium pentoxide or dichlorodicyanoquinone.These oxidizing agents convert the NO into NO₂. By using a suitableoxidizing agent in combination with a polyarylene ether, the filtermaterial, the filter and the process are also suitable for removing NOfrom gases and liquids.

The invention additionally provides, therefore, a filter material and afilter for removing NO and NO₂ from gases and liquids, the filtermaterial or the filter comprising a polyarylene ether and an oxidizingagent having a redox potential of at least 0.96 V SHE.

The invention additionally provides a process for producing NO-free andNO₂ -free gases or liquids, which comprises bringing the gas to bepurified or the liquid to be purified into contact with an oxidizingagent having a redox potential of at least 0.96 V SHE and with amaterial which comprises a polyarylene ether.

The filter material, the filter and the process for removing NO and/orNO₂ can be employed with all liquids and gases which contain oxides ofnitrogen. They can be employed, for example, in filter masks, inair-conditioning systems, in automobiles (e.g. air filters, exhaustfilters), for removing nitrogen oxides produced in combustion (e.g.flue-gas purification), and also for removing and detoxifying oxides ofnitrogen in liquids.

Polyarylene ethers can also be employed as a suspension or solution forremoving NO and/or NO₂ from gases. For example, suspensions ofpolyarylene ether may consist of finely divided polyarylene ether inwater. Solutions of polyarylene ethers can be prepared, for example,with aromatic solvents such as toluene or nonaromatic solvents such aschloroform. For removing oxides of nitrogen, a gas to be purified ispassed through the liquid.

The removal of NO and/or NO₂ from a liquid can, for example, involve thepolymer being suspended in the liquid (extractive stirring method) orbeing passed through a column which is packed with the polymer (columnmethod).

It is advantageous for the removal of NO₂ or NO from a gas or a liquidto use a filter material having a large surface area, for example aporous powder or a porous fiber.

DETAILED DESCRIPTION

Turning now to FIG. 1 of the Drawing, a preferred embodiment of system10 for utilizing a filter cartridge 15 of this invention includes, onthe cartridge-inlet side, a gas mixing system and flow control system 21wherein a commercial analytical gas mixture containing a minor amount ofNO₂ (e.g. 538 ppm) and synthetic air is diluted, so that the NO₂ contentof the resulting gas stream 11 is decreased and/or the pressure of gasstream 11 is controlled (flow control system 21 can include a pressurereduction valve). Gas stream 11 is passed through cartridge 15, which ispacked with a filter material 13 comprising a poly(phenylene oxide) suchas poly-para 2,6-dimethylphenylene oxide!. The filter material 13 can beparticulate, e.g. in the form of finely ground granules. The gas stream19 which has passed through cartridge 15 is analyzed, on the gas-outletside 17, by a suitable measuring instrument for both NO and NO₂ content,e.g. an NO/NO₂ chemoluminescence device 23 which can detect <1 ppm ofeither of these gases. The content of NO₂ (and optionally NO) in gasstream 19 can thereby measured over a period time, e.g. several hours.

FIG. 2 illustrates how the filter cartridge 15 of FIG. 1 can be utilizedin an artificial respiration system 30 comprising a breathing mask 31,filter 15, and pressure reduction valve 35, filter 15 purifies theintake air flow 33 upstream of mask 31, so that an essentially NO₂ -freegas mixture can be inhaled from breathing mask 31. The pressurereduction valve 35 controls the volume flow so that it will be in therange of 0.001 to 1000 l/min, preferably between 0.01 and 250 l/min.

EXAMPLES

1) A gas mixture of 100 ppm of NO₂ with nitrogen was produced in a gasmixing system consisting of flow controllers (type 1259C) and thecorresponding control instrument (type 247C, both from MKS Instruments,81829 Munich, Federal Republic of Germany) by diluting a commercialanalytical gas mixture (538 ppm of NO₂ in synthetic air, MesserGriesheim GmbH, Specialty Gases Plant, 47009 Duisburg, Federal Republicof Germany) and was passed at room temperature (25° C.) through a filtercartridge packed with poly-para 2,6-dimethylphenylene oxide!,abbreviated to PPO, in the form of finely ground granules (averageparticle diameter D₅₀ : about 50 μm). The absorption path ischaracterized by the following parameters:

    ______________________________________                                        Internal diameter of the filter                                                                       2     cm                                              cartridge:                                                                    Mass of PPO employed:   15    g                                               Height of PPO bed:      9     cm                                              Gas throughput:         25    l/h                                             Gas flow rate:          2.2   cm/s                                            ______________________________________                                    

After passing through the filter cartridge, the gas was analyzed for itscontent of NO and NO₂ by passing it into an NO/NO₂ chemoluminescencemeasuring instrument (type CLD 700 El Ht, Eco Physics AG, Durnten,Switzerland; minimum detection limit 0.1 ppm, linearity ±1% end-scaledeflection) with the measurement range setting 0-100 ppm. The filteraction for NO₂ starts straight away. Within a period of 2 hours, the NO₂concentration was below the detection limit of 1 ppm. For a further 18hours, the concentration of NO₂ remained below the MAC value of 5 ppm.

2) A gas mixture of 500 ppm of NO₂ in synthetic air was passed as inExample 1 (measurement range 0-1000 ppm) through a filter cartridge andanalyzed. The absorption path is characterized by the followingparameters:

    ______________________________________                                        Internal diameter of the filter                                                                      2      cm                                              cartridge:                                                                    Mass of PPO employed:  5      g                                               Height of PPO bed:     3      cm                                              Gas throughput:        48     l/h                                             Gas flow rate:         4.25   cm/s                                            ______________________________________                                    

The filter was fed with gas until the concentration of NO₂ passedthrough was about 80% of the initial concentration. The filter capacitycalculated from this was 18% (percent by weight) based on NO₂

3) With a gas mixture with 500 ppm of NO₂ in helium, produced as inExample 1 (but with an analytical gas mixture comprising 600 ppm of NO₂in helium; measurement range 0-1000 ppm), the concentration of NO₂remained below the detection limit for 20 minutes. After 16 hours, 40ppm were measured, i.e. after this time the filter still has anefficiency of more than 90%.

4) A gas mixture of 500 ppm of NO₂ in synthetic air was passed as inExample 1 at room temperature (25° C.) through a filter cartridge whichwas packed with a polyarylene ether (Blendex XHPP 820, GE Plastics,U.S.A.) in granular form (average particle diameter about 500-800 μm)The absorption path is characterized by the following parameters:

    ______________________________________                                        Internal diameter of the filter                                                                      2      cm                                              cartridge:                                                                    Mass of PPO employed:  5      g                                               Height of PPO bed:     4.5    cm                                              Gas throughput:        48     l/h                                             Gas flow rate:         4.25   cm/s                                            ______________________________________                                    

At the beginning of the experiment, the NO₂ concentration remained belowthe detection limit for 5 minutes. After 3 hours, 75 ppm of NO₂ wereable to pass through the filter (measurement range 0-1000 ppm).

We claim:
 1. A filter material for removing NO and NO₂ from gases orliquids, comprising a polyarylene ether and an oxidizing agent having aredox potential of at least 0.96 V SHE.
 2. A filter for treating anNO_(x) containing gas, wherein NO_(x) includes NO₂, comprising a filtercartridge and, contained therein, a filter material which is constructedand arranged to decrease the amount of NO₂ in said NO_(x) -containinggas, said filter material comprising a filter material as claimed inclaim
 1. 3. An artificial respiration system for providing a gas mixtureflow, said system comprising a pressure reduction valve, for providing agas mixture flow of from 0.001 to 1000 l/min, and a filter as claimed inclaim
 2. 4. An artificial respiration system as claimed in claim 3,wherein said system includes a breathing mask, and said filter cartridgeis arranged to be upstream of the breathing mask.
 5. A filter comprisinga filter cartridge and, contained therein, a filter material of claim 1.6. A filter as claimed in claim 2, wherein the filter material comprisesa polyarylene ether having polyarylene ether units of the formula--Ar--O--, wherein Ar is a monocyclic aromatic group having two bondingsites.
 7. A filter as claimed in claim 6, wherein said monocyclicaromatic group is benzene or pyridine which is unsubstituted or issubstituted with at least one C₁ -C₁₈ -alkyl.
 8. A filter as claimed inclaim 7, wherein said aromatic group is 2,6-dimethylphenylene.
 9. Anartificial respiration system as claimed in claim 3, wherein the filtercontains a polyarylene ether having an arylene unit of the formula--Ar--O--, wherein Ar is benzene or pyridine which is unsubstituted orsubstituted with at least one C₁ -C₁₈ -alkyl.
 10. A filter material asclaimed in claim 1, wherein the material comprises a polyarylene etherhaving an arylene unit of the formula --Ar--O--, wherein Ar is benzeneor pyridine which is unsubstituted or substituted with at least one C₁-C₁₈ -alkyl.
 11. A process for treating an NO_(x) -containing gas orliquid, wherein NO_(x) includes NO₂, to decrease the amount of NO₂therein, which comprises bringing the gas or liquid to be treated intocontact with a material which comprises a polyarylene ether, and whereinthe amount of NO₂ in said gas or liquid to be treated ranges from 1 ppbto 50 volume %.
 12. The process as claimed in claim 11, wherein the gasor liquid to be treated reacts chemically with the polyarylene ether.13. The process as claimed in claim 12, wherein the polyarylene ether isnot converted to a volatile product.
 14. The process as claimed in claim11, wherein the gas or liquid to be treated contains oxygen.
 15. Theprocess as claimed in claim 11, wherein the gas or liquid to be treatedcontains nitrogen or air.
 16. The process as claimed in claim 11,wherein the NO₂ of said gas or liquid to be treated is in equilibriumwith N₂ O₄.
 17. The process as claimed in claim 11, wherein thepolyarylene ether has arylene ether units of the formula --Ar--O--, andwherein Ar is benzene or pyridine which is unsubstituted or substitutedwith at least one C₁ -C₁₈ -alkyl.
 18. The process as claimed in claim17, wherein said polyarylene ether comprises poly(2,6-dimethylphenyleneoxide).
 19. The process as claimed in claim 11 wherein the amount of NOin said gas or liquid to be treated ranges from 1 ppb to 60 volume %.20. The process as claimed in claim 11, wherein said gas or liquid to betreated contains both NO and NO₂, and wherein the amounts of both NO andNO₂ are decreased by bringing the gas or liquid to be treated intocontact with a polyarylene ether and an oxidizing agent having a redoxpotential of at least 0.96 V SHE.
 21. A method for improving the lungfunction of a patient comprising the steps of:providing a flow of lungfunction-improving gas containing oxygen and NO and at least theinherent content of NO₂ resulting from oxidation of NO, and selectivelydecreasing the amount of NO₂ in said lung function-improving gas bybringing said flow of lung function-improving gas into contact with apolyarylene ether before delivering said gas to a patient.